Inventive concepts relate to a pixel of a photodetecting device, and more particularly, to a unit pixel capable of achieving full initialization of a floating diffusion area.
A photodetecting device converts an image signal input through a lens into an electrical signal, and currently, popular photodetecting devices include a complementary metal-oxide semiconductor (CMOS) image sensor (CIS). A photodetecting device includes two-dimensionally arranged unit pixels as a pixel array for converting an image signal into an electrical signal. Each of the unit pixels includes a photoelectric conversion device for detecting an image signal, e.g., a photodiode, and a driving unit for amplifying and transmitting an output of the photoelectric conversion device.
The driving unit includes a transmission transistor for transmitting charges corresponding to an image signal detected by the photoelectric conversion device to a floating diffusion area, and a conversion transistor for converting the charges stored in the floating diffusion area into an electrical signal corresponding to the charges. Here, the conversion transistor is also referred to as a source follower transistor.
A photodetecting device detects an image signal and converts the image signal into an electrical signal corresponding to the image signal, and then the converted electrical signal may not directly be stored as image data but may be processed as described below before being stored.
In order to detect an image signal and to convert the image signal into an electrical signal, first, a floating diffusion area is initialized by using a voltage and then a first voltage V1 corresponding to the initialized floating diffusion area may be recognized. After that, charges detected by a photodiode are transmitted to the floating diffusion area and a second voltage V2 corresponding to the charges transmitted to the floating diffusion area may be recognized. Image data corresponding to the image signal detected by the photodiode corresponds to the difference between the first and second voltages V1 and V2 (V1-V2).
In order to initialize the floating diffusion area, each of the unit pixels further includes a reset transistor. The reset transistor transmits an initialization voltage applied to a first terminal of the reset transistor to the floating diffusion area through a second terminal of the reset transistor in response to a reset control signal applied to a gate of the reset transistor. Here, the initialization voltage refers to voltage power for initializing the floating diffusion area.
In general, an initialization voltage source connected to a first terminal of a reset transistor may also be connected to a first terminal of a conversion transistor. Thus, in some cases, sufficient charges may not be provided to a floating diffusion area. That is, in some cases, a floating diffusion area may not be completely initialized by using only a reset transistor. Accordingly, a loss occurs while converting an image signal applied from an external unit to an electrical signal. If an electrical signal having a loss is reproduced as an image signal, an image signal different from an original image signal may be displayed.